programming style.

Recoded many routines to incorporate better error checking, and/or
better versions of the same function found elsewhere
(e.g. bltinmodule.c or ceval.c).  In particular,
Py_Number_{Int,Long,Float}() now convert from strings, just like the
built-in functions int(), long() and float().

Sequences and mappings are now safe to have NULL function pointers
anywhere in their tp_as_sequence or tp_as_mapping fields.  (A few
places in other files need to be checked in too.)

Renamed PySequence_In() to PySequence_Contains().